A conventional interchangeable lens unit adopts a mechanism that mechanically drives an optical system such as a focus lens and a zoom lens. Against this, in recent years, an interchangeable lens unit is suggested that drives an optical system with an electrically driven actuator such as a motor (for example, see Patent Document 1).
On the other hand, some of cameras built-in lens are mounted with a step zoom function for being capable of setting a predetermined zoom focal length in order to enable photographing with predetermined variable magnification (for example, see Patent Document 2).